The present invention relates to the method for enhancing the recovery of petroleum from an oil bearing formation.
In the recovery of oil from reservoirs, the use of primary production techniques (i.e., the use of only the initial formation energy to recover the crude oil) followed by the secondary technique of water flooding, recovers only a portion of the original oil present in the formation.
Moreover, the use of certain enhanced oil recovery (EOR) techniques are also known in the art. These techniques can generally be classified as either a thermally based recovery technique i.e., utilizing steam, or a gas drive method that can be operated in a miscible or non-miscible manner.
Methods which employ steam are effective in the enhanced recovery of oil because the steam heats the formation, lowers the viscosity of the oil and thus, enhances the flow of the oil towards a production well. Moreover, these methods have become preferred methods for the enhanced recovery of low gravity, high viscosity oils and because steam can cost effectively provide heat to such oils.
However, in these steam based techniques, it is common that the steam will find short cut pathways from the injection well to some of the producing wells, thus bypassing, oil which is present in the zone between the injection well and the production well. Also, after the initial steam injection breakthrough at the production well, the steam injection preferentially follows the path of the breakthrough. These pathways can take the form of channels in the formation or of gravity override in the upper portion of the oil bearing stratum. Gravity override results from the lower density and viscosity of the steam vapor compared to liquid oil and water. Thus, the total amount of the formation that is swept by the steam injection is limited.
Various methods have been proposed to mitigate the loss of steam flow and heating value in the formation. For example, a number of commercial surfactants have been injected along with the steam to create a steam-foam flood. Surfactants form a foam that inhibits the flow of the steam into that portion of the formation containing only residual oil saturation and serves to physically block the volumes through which the steam is shortcutting. This forces the steam to move the recoverable hydrocarbons from the lesser portion of the reservoir towards the production well.
In addition, various inert and non-condensible gases have been added to the steam, both in the presence and absence of foaming surfactants in order to enhance and maintain the oil-driving force within the formation.
Examples of steam-foam processes can be found in U.S. Pat. Nos. 4,086,964; 4,445,573; 4,393,937; 4,161,217; and 4,085,800.
In particular, certain alkyl aromatic sulfonates have been employed as a surfactant within the steam-foam drive systems. See for example, Canadian Patent 1,247,850.
In general, these foaming agents are manufactured, transported, and stored, in a concentrated form prior to being employed within the field.
It has now been discovered that when higher average molecular weight alkyl aromatic sulfonates are subjected to extended storage (i.e., storage extending from days to several months, generally at temperatures about 70.degree. F.) and/or exposure to cold temperatures (i.e., temperatures about 20.degree. F.), they will increase in viscosity and become a gel-like mixture (i.e., a Brookfield viscosity greater than or equal to about 10,000-15,000 cp). This dramatic increase in viscosity makes field handling of the concentrate very difficult.
Thus, the need still exists for a composition which is effective in enhanced recovery procedures, particularly with steam, and which can be more easily handled within the field despite being subjected to storage for extended periods or exposure to low temperatures.
Accordingly, it is the object of the present invention to provide an effective method for enhancing the recovery of petroleum from oil-bearing formations.
It is a further object to provide a mixture which can be effectively employed with steam foam system and which is not subject to an undesirable increase in viscosity when stored, particularly for extended periods and/or after exposure to low temperatures.
These and further objects will become apparent from the specification and claims which follow.